In recent years, special attention has been given to a synchrotron radiation (SR) apparatus as a short-wavelength light source for manufacturing semiconductor devices. Electrons accelerated nearly to the light velocity are stored in a predetermined orbit, so that SR lights generated in a tangential direction when electrons are bent in magnetic field or the like are taken out as light output. The SR light is generated with continuous spectra over a wide wavelength range.
The SR apparatus is utilized as an X-ray lithography light source for the manufacture of semiconductor devices and as a monochromatic X-ray source for the structural analysis of a substance, the elemental analysis, the medical and measuring purpose such as an X-ray microscope, etc.
In the SR apparatus, electrons are stored in the electron orbit to thereby generate SR light, but it cannot be said that the strength of the SR light at a desired wavelength is always sufficient. In general, sufficiently short-wavelength light cannot be generated if electrons cannot be accelerated to 1 GeV or more. To this end, generally, the apparats has an electron orbit shaped like a race track, a circle, or the like, with a radius of 10 meters or more. However, if such a large light source is used, the light source becomes too large as a light source for the production of semiconductor devices. Therefore, a smaller-sized short-wavelength light source has been demanded. Various proposals for improving the usefulness of the SR apparatus have been made.
There has ben a proposal in which a light guide for reflecting radiation is provided so as to surround the outer circumference of an electron orbit of an electron storage ring to thereby constitute the Photon Storage Ring. When, for example, the electron orbit is circular, a barrel-like or cylindrical mirror surface which is concave in section encloses the electron orbit. Radiation generated from the electron orbit is reflected on the light guide and stored in the light guide. In this way, an intense radiation can be taken out.
When the electron orbit in the electron storage ring is truly circular and the light guide forms a concentric circle with respect to the electron orbit, monochromatic light can be taken out by interference of lights if the radius of curvature of the light guide is set to a specific value.
A free electron laser can be formed by inducing emission through interactions between electrons and the light stored in the light storage ring.
However, suffering from various restrictions, the wavelength of the light generated by these means cannot be made sufficiently short.